Active sound reduction systems exist in various technical implementations in different fields wherever it is desired to cancel noise or any undesired sound emissions of a primary sound source. The term “active” refers to any means which can react to the sound to be cancelled, providing a number of advantages over passive means like insulating mats, noise-absorbent walls or casings etc. The most common active sound reduction system is the so-called Active Noise Control (ANC) which aims at canceling unwanted noise by introducing an additional sound field that destructively interferes with the original noise from the primary sound source. This additional sound wave is sometimes referred to as “anti-noise”. Provided that amplitude and phase of the anti-noise field are matched to the noise emitted by the primary sound source, the residual sound resulting from the superposition of the sounds from the primary sound source and the secondary sound source generating the anti-noise should be cancelled completely. Although the basic idea of active noise control has already been developed over some decades, serious successful applications are few. The reason is that the theoretical principle of canceling noise by anti-noise is not often successful in practice, since there is a number of determining conditions which are often not fulfilled. Just to mention one example, reflections of the sound emitted by the primary sound source also have to be cancelled to extinguish the original sound completely, which can only be achieved by great operation expense and effort.
In recent years the research has been concentrated on applications offering well-defined conditions to use an active sound reduction system of the above kind successfully. One example for such an application is the cancellation of snoring sounds emitted by a human being, which can be a major nuisance for a sleeping partner. The sound associated with snoring is caused by the vibration of a part of the upper respiratory tract. This vibration can occur in the soft palate, tongue, tonsils or epiglottis, of which palatal snoring is by far the most common. In ten percent of snorers, the upper airway suffers a partial or full collapse, resulting in cessation of respiratory airflow which leads to premature arousal from sleep. If this obstruction lasts at least ten seconds and happens repeatedly, the snorer suffers from Obstructive Sleep Apnea (OSA). This serious condition is associated with hypertension, ischemic heart disease and stroke, as well as industrial accidents, driving fatalities and lost productivity due to daytime sleepiness. It results from the above that snoring is more than an annoyance but also represents a serious medical problem which is very common in the general population.
There have been attempts to apply Active Noise Control (ANC) for the attenuation of snoring sounds emitted by a human being in a typical environment like a bedroom. U.S. Pat. No. 5,844,996 refers to an active electronic noise suppression system and a corresponding method for reducing snoring noise, using a reference microphone mounted above the snoring person for receiving the sound from this person as a primary sound source, and a number of error microphones which are arranged to receive the residual sound resulting from the superposition of the sounds from the snorer and a speaker which generates an attenuating sound which is superposed with the sound emitted by the snorer. The speaker is controlled according to a reference signal corresponding to the sound received by the reference microphone and an output error signal received by the error microphone, corresponding to the residual sound. The system aims at attenuating the residual sound to zero so that the noise in the area wherein the error microphone is located is cancelled completely. With other words, the control signal for the speaker producing the attenuating sound is calculated by processing the reference signal by the reference microphone in such a way that the residual signal will be minimized, to create a “quiet zone” in a certain area wherein a sleeping partner is located.
Like other applications, the above mentioned application of active noise control suffers from certain deficiencies, coursed by the preconditions given by the environment, like reflections of the walls and so on. One resulting problem is that a quiet zone, which is desired to be as large as possible, is very small. The reason is that the phase opposition, which is a condition for cancellation of the primary and secondary sound, is lost easily when the bed partner moves in any direction, causing a phase mismatch. As the movements of a human being in sleep are not controllable, there is no practical way to keep the sleeping partner within a very small quiet zone generated by the known ANC-System. The effectivity of this system is therefore small, as well as its usefulness in practice.
It is therefore an object of the present invention to improve the effectivity of an active sound reduction system and method as described above. Stated more precisely, one object of the present invention lies in providing a system and method for active sound reduction which enlarges the size the quiet zone in which optimal attenuation of the sound emitted by the snorer takes place.